1. Field of the Invention
The present invention relates to a semiconductor laser device for single-longitudinal-mode operation.
2. Description of the Related Art
Many of presently available semiconductor laser devices of heterostructure usually operate in multimode to emit laser light. To obtain a single-mode oscillation spectrum, distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) and so on have been proposed. In accordance with semiconductor lasers of this type, on part of a semiconductor substrate a diffraction grating consisting of periodic grooves is formed which feeds back or reflects only a specific wavelength to obtain the single-longitudinal-mode operation, or the single wavelength laser oscillation.
In accordance with the semiconductor lasers using the diffraction grating, however, it is difficult to realize a good sideband suppression ratio. If the side-band suppression ratio is poor, the characteristics of the single-longitudinal-mode operation of the semiconductor laser will be degraded in a high-speed modulation operation. Moreover, with existing manufacturing technology, it is extremely difficult to form the diffraction grating in the semiconductor lasers of that type with as high precision as desired and as high manufacturing yield as desired. One of the reasons is complication of the manufacturing process due to practice of the holographic interferometer system or two-beam interference exposure system. To form a diffraction grating whose shape is controlled with relatively high precision, it is required to use the two-beam interference exposure technique which needs a highly complex control system. This will lower the efficiency of manufacture of semiconductor lasers. Another reason is a break in the shape of the diffraction grating due to high temperatures involved in the epitaxial growth process. Even if a diffraction grating closely controlled in shape is initially obtained in manufacture of a semiconductor laser, the diffraction grating will not be able to retain its shape due to high temperatures involved in epitaxially growing single-crystal semiconductor materials on the diffraction grating.
A semiconductor laser device that oscillates at a single wavelength without using a diffraction grating is described by J. P. Van der Ziel at al in Appl. Phys. Lett., 50 (1987) page 1313. The described semiconductor laser includes an active layer which is doped or added with a rare earth element, such as ytterbium (Yb) or erbium (Er), at high impurity concentration. When the semiconductor laser is excited to emit light, the rare earth element emits light spontaneously because of the transition of f electrons between their orbits. The single-mode laser oscillation is made possible by the use of the spontaneous emission from the rare earth element.
With this type of semiconductor laser, however, to effectively utilize the spontaneous emission from the rare earth element itself for oscillation of laser light, the active layer must be doped with impurities of an rare earth element in great quantity. The doping with a large quantity of impurity of rare earth element will adversely affect the crystalline quality of the active layer so that the property of interface between the active layer and an epitaxially grown semiconductor material formed thereon is deteriorated. The degradation of the crystalline quality of the active layer would increase the oscillation threshold of the semiconductor laser, thus deteriorating its basic characteristics for oscillation. Furthermore the doping of the active layer with a great quantity of rare-earth impurity will result in undesired precipitation of impurity in the multilayer structure of a semiconductor laser. This fact will lead to phenomena such as an increase in threshold current and decreases in light-emitting efficiency and lifetime.